U.S. patent application number 10/742836 was filed with the patent office on 2004-07-15 for method of analyzing and displaying blood volume using myocardial blood volume map.
This patent application is currently assigned to YD, Ltd.. Invention is credited to Ito, Hiroshi, Nakajima, Yasuhiro.
Application Number | 20040138567 10/742836 |
Document ID | / |
Family ID | 32708407 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040138567 |
Kind Code |
A1 |
Ito, Hiroshi ; et
al. |
July 15, 2004 |
Method of analyzing and displaying blood volume using myocardial
blood volume map
Abstract
A contrast echo image of the myocardium and a cardiac chamber
obtained by use of ultrasound diagnostic equipment is analyzed so
as to determine the volume of blood within the myocardium. The
contrast echo image is divided into a plurality of calculation
regions, each being considered to have a uniform acoustic field
intensity and covering a portion of a myocardium image region and a
corresponding portion of a cardiac chamber image region. For each
calculation region, a difference in intensity between the
myocardium image region and the cardiac chamber image region is
obtained, and the myocardium image region is colored in accordance
with the difference.
Inventors: |
Ito, Hiroshi;
(Toyonaka-city, JP) ; Nakajima, Yasuhiro;
(Ikoma-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
YD, Ltd.
Ikoma-city
JP
630-0243
ME, Co., Ltd.
Nagano-city
JP
388-8020
Hiroshi ITO
Toyonaka-city
JP
560-0085
|
Family ID: |
32708407 |
Appl. No.: |
10/742836 |
Filed: |
December 23, 2003 |
Current U.S.
Class: |
600/458 |
Current CPC
Class: |
A61B 8/481 20130101 |
Class at
Publication: |
600/458 |
International
Class: |
A61B 008/14 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2002 |
JP |
2002-379760 |
Claims
What is claimed is:
1. A blood-volume analysis-display method of analyzing a contrast
echo image of the myocardium and a cardiac chamber so as to
determine and display the volume of blood within the myocardium,
the contrast echo image being obtained by use of ultrasound
diagnostic equipment after injection of a microbubble ultrasound
contrast agent into blood vessels, and including a myocardium image
region corresponding to the myocardium and a cardiac chamber image
region corresponding to the cardiac chamber, the method comprising:
(a) finely diving the contrast echo image into a plurality of
calculation regions, each being considered to have a uniform
acoustic field intensity and covering a portion of the myocardium
image region and a corresponding portion of the cardiac chamber
image region; (b) obtaining, for each calculation region, a
difference in intensity between the myocardium image region and the
cardiac chamber image region, and determining the volume of blood
within the myocardium on the basis of the difference; and (c)
coloring the myocardium image region in accordance with the
determined blood volume.
2. A blood-volume analysis-display method according to claim 1,
wherein for each calculation region, a power P is first obtained by
the following equation: 10 log.sub.10P=I.sub.1-I.sub.2 where
I.sub.1 represents the intensity of the myocardium image region,
I.sub.2 represents the intensity of the cardiac chamber image
region; and subsequently, a blood volume V (unit: ml/100 g: blood
volume) in the myocardium is calculated by the following equation:
V=100.times.P (ml/100 g).
3. A blood-volume analysis-display method according to claim 1,
comprising: (a) tracing the endocardial and the epicardial to
thereby form an endocardial line and an epicardial line,
respectively; (b) drawing an inner line and an outer line on the
inner and outer sides, respectively, of the endocardial line in
such a manner that each of the inner line and the outer line
extends along the endocardial line while separated therefrom by a
predetermined distance; (c) drawing two boundary lines at a
predetermined interval in the depth direction in order to obtain a
region that is considered to have a uniform acoustic field
intensity; (d) defining a cardiac chamber calculation region, which
is a region surrounded by the endocardial line, the inner line, and
the boundary lines, and a myocardium calculation region, which is a
region surrounded by the endocardial line, the outer line, and the
boundary lines; (e) subtracting the average intensity of the
cardiac chamber calculation region from the average intensity of
the myocardium calculation region to thereby obtain a luminous
difference; (f) selecting a color corresponding to the luminous
difference from a color table, and coloring with the selected color
the entirety of a myocardium image region sandwiched between the
acoustic field lines; and (g) repeating the steps (c) to (f), while
shifting the boundary lines in the depth direction by a shifting
distance not greater than the predetermined interval.
4. A blood-volume analysis-display method according to claim 3,
wherein the shifting distance is a smaller than the predetermined
interval.
5. A blood-volume analysis-display method according to claim 4,
wherein the shifting distance is about one tenth of the
predetermined interval.
6. A blood-volume analysis-display method according to claim 1,
comprising: (a) tracing the endocardial and the epicardial to
thereby form an endocardial line and an epicardial line,
respectively; (b) drawing an inner line on the inner side of the
endocardial line in such a manner that the inner line extends along
the endocardial line while separated therefrom by a predetermined
distance; (c) drawing two boundary lines at a predetermined
interval in the depth direction in order to obtain a region that is
considered to have a uniform acoustic field intensity; (d) defining
a cardiac chamber calculation region, which is a region surrounded
by the endocardial line, the inner line, and the boundary lines,
and a myocardium calculation region, which is a region surrounded
by the endocardial line, the epicardial line, and the boundary
lines; (e) calculating the difference between the intensity of each
pixel within the myocardium calculation region and the average
intensity of the cardiac chamber calculation region to thereby
obtain a luminous difference for each pixel, and coloring each
pixel with a color corresponding to the luminous difference, the
color being selected from a color table; and (f) repeating the
steps (c) to (e), while shifting the boundary lines in the depth
direction by a shifting distance not greater than the predetermined
interval.
7. A blood-volume analysis-display method according to claim 3,
further comprising: drawing first and second buffer lines on the
inner side and outer side, respectively, of the endocardial line in
such a manner that each of the buffer lines extends along the
endocardial line while being separated from the endocardial line by
a predetermined distance; and defining, as the cardiac chamber
calculation region, a region surrounded by the boundary lines, the
first buffer line, and the inner line; and defining, as the
myocardium image region, a region surrounded by the boundary lines,
the second buffer line, and the outer line.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a contrast echo image
analysis method employed for detection or treatment of ischemic
heart diseases or similar diseases, in which a contrast echo image
of the myocardium and cardiac chambers--which image is obtained by
use of ultrasound diagnostic equipment after injection of a
microbubble ultrasound contrast agent into blood vessel--is
analyzed so as to visually and quantitatively determine the volume
of blood flowing through the myocardium on the basis of intensity
values of the myocardium region and the cardiac chamber regions of
the image.
[0003] 2. Background Art
[0004] X-ray diagnostic equipment, X-ray CT equipment, MRI
equipment, or nuclear medical diagnostic equipment enables
diagnosis of interior tissues of a human body without surgical
operation, but such equipment involves the following problems: a
subject is exposed to X-rays or nuclear radiation, and examination
requires a long period of time. Meanwhile, catheterization is an
invasive examination technique in which a catheter is inserted into
a blood vessel. In contrast, ultrasound diagnostic equipment, which
employs ultrasound waves, causes virtually no adverse effects on
body tissues of a subject--which would otherwise be incurred
through exposure of the subject to X-rays, nuclear radiation, or
magnetic field during use of the aforementioned equipment; i.e.,
ultrasound diagnostic equipment is non-invasive diagnostic
equipment. Ultrasound diagnostic equipment attains real-time image
display, involves low risk even when repeatedly used within a short
period of time, and enables examinations to be completed within a
short period of time. Furthermore, ultrasound diagnostic equipment
is small in size and is inexpensive, and thus can be used in any
hospital. In addition, such ultrasound diagnostic equipment allows
a medical doctor to readily and thoroughly examine a region of
interest (ROI) while directly operating the equipment.
[0005] When ultrasound diagnostic equipment is used, an ultrasound
probe is placed on the body surface in order to receive reflection
waves of ultrasound waves emitted from a transducer, and
tomographic images of body tissues are obtained on the basis of the
received reflection waves. Therefore, for example, the state of the
heart, the abdomen, or the mammary gland, or movement of a fetus
within the uterus can be observed in real time. Meanwhile, through
use of the power Doppler method, imaging of the blood flow can be
performed.
[0006] The blood that has circulated throughout the body flows
through veins to the right atrium and the right ventricle, and then
flows from the right ventricle through arteries to the lungs. The
blood is oxygenated in the lungs. The blood that has been
oxygenated in the lungs passes through the left atrium and the left
ventricle, and flows through arteries to the entire body. During
this circulation, a portion of the thus-oxygenated blood is fed to
coronary arteries, which are branching from the aorta. The coronary
arteries, which cover the myocardium while forming a web-like
network, supply oxygen and nutrients, which energize the heart, to
the myocardium. When the coronary arteries are occluded or stenosed
due to formation of thrombi or occurrence of coronary artery
disorder, the amount of the blood supplied to the heart is reduced,
causing adverse effects on action of the heart. When the coronary
arteries are occluded, and necrosis of myocardial cells occurs, the
contraction force of the myocardial cells is lost. This symptom is
called "myocardial infarction." When the amount of the blood
supplied to the myocardium is reduced as a result of narrowing of
the coronary arteries, although occlusion of the arteries does not
occur, the heart is adversely affected. This symptom is called
"angina pectoris." Myocardial infarction and angina pectoris are
collectively called "ischemic heart diseases."
[0007] In the case where the heart is examined by use of ultrasound
diagnostic equipment, a contrast agent is injected via a vein in
order to evaluate blood perfusion, because the intravenous
injection of a contrast agent is less invasive. In diagnosis by use
of a contrast agent, change with time in spatial distribution of
the contrast agent in a site to be diagnosed is observed on the
basis of an increase in an intensity-enhanced area and an increase
in intensity. In addition, there is obtained the transit time
between the injection of the contrast agent and the arrival thereof
to a region of interest (ROI), as well as the time intensity curve
(TIC), which represents change with time in intensity of an echo
image of the ROI captured by use of a contrast agent.
[0008] Ultrasound waves (echo signals) reflected from an organ of a
living organism tend not to exhibit non-linear behaviors. However,
echo signals obtained by use of an ultrasound contrast agent
predominantly containing microbubbles include non-fundamental wave
components attributable to non-linear behaviors. Therefore, when
only the non-fundamental wave components are separated from the
echo signals and detected, an organ of a living organism and a
cavity (e.g., the interior of a blood vessel or the cardiac
chambers of the heart) can be observed at a high contrast ratio in
an image generated by a contrast agent.
[0009] The aforementioned contrast echo method is disclosed in, for
example, Japanese Patent Application Laid-Open (kokai) No.
11-155858 (title of the invention: "Ultrasound Diagnostic Equipment
and Ultrasound Diagnostic Method") and Japanese Patent Application
Laid-Open (kokai) No. 2001-178722 (title of the invention:
"Ultrasound Diagnostic Equipment and Ultrasound Diagnostic
Method"). The former patent document discloses a method for
obtaining a more effective contrast echo image, in which the
transmission sound pressure of ultrasound pulses is optimized in
order to increase the intensity of an image obtained by use of a
contrast agent. The latter patent document discloses a method and
equipment for reducing labor or burden of an operator when carrying
out a contrast echo method employing an ultrasound contrast agent
predominantly containing microbubbles. This patent document
proposes a method for informing a diagnostician of vanishment of
microbubbles in the form of sound information, by use of a
speaker.
[0010] In the case where the heart is examined by use of ultrasound
diagnostic equipment and in accordance with the contrast echo
method employing an ultrasound contrast agent predominantly
containing microbubbles, a high level of skill is required for
diagnosing ischemic heart diseases (e.g., myocardial infarction and
angina pectoris) on the basis of the resultant contrast echo
images. The reasons for this are described below.
[0011] When a microbubble ultrasound contrast agent is injected
into blood vessels, and an ultrasound probe is placed on the body
surface to thereby apply to a region of interest (ROI) ultrasound
waves transmitted from the probe, the ultrasound waves are
reflected by microbubbles contained in the contrast agent.
Ultrasound waves of high energy are reflected from a site where
blood containing the contrast agent is present. When received
reflective waves are converted into intensity and an image is
displayed on the basis of the intensity, a portion of the image
corresponding to such a site is displayed with a higher intensity;
i.e., as a bright region. A site to which the
contrast-agent-containing blood is not supplied in a sufficient
quantity is displayed with a lower intensity; i.e., as a dark
region. Therefore, whether or not the blood is sufficiently
supplied to the myocardium can be determined through comparison of
the intensity of the myocardium at different locations.
[0012] Ultrasound waves transmitted from the probe attenuate with
the distance between the body surface and a relevant site of the
body; i.e., the depth of the site. Further, since the probe focuses
ultrasound waves on a relevant site, the energy level at that site
becomes higher than that of other sites. Moreover, in the case
where a color image of the heart is obtained by means of the power
Doppler method or the B-mode method, the color of image regions
corresponding to the cardiac chambers bleeds into an image region
corresponding to the myocardium. Therefore, difficulty is
encountered in correctly determining the state of perfusion of
blood on the basis of contrast echo images of the myocardium and
the cardiac chambers.
[0013] As described above, in the case of a conventional contrast
echo image, intensity varies from site to site because of depthwise
attenuation of ultrasound waves and energy maximization at a site
where ultrasound waves are focused. However, in the conventional
image processing technique, intensity measurement is performed
without correction of variation in intensity, which variation
occurs due to the difference in acoustic field or focusing of
ultrasound waves. Therefore, in the conventional technique,
difficulty is encountered in determining whether the difference in
intensity between different sites in the cardiac chambers or the
myocardium is caused by the difference in acoustic field or by the
difference in perfusion.
[0014] Meanwhile, diagnosing the state of perfusion of the cardiac
chambers or the myocardium on the basis of a contrast echo image of
the heart depends on the skill of a diagnostician, and an unskilled
diagnostician encounters difficulty in correctly diagnosing such
perfusion in a simple manner. This is because image processing has
not conventionally been performed objectively, and quantitative
analysis of the contrast echo image has not been carried out.
SUMMARY OF THE INVENTION
[0015] In view of the foregoing, an object of the present invention
is to provide a method for analyzing blood perfusion within the
myocardium on the basis of a contrast echo image obtained by use of
ultrasound diagnostic equipment, to thereby provide a diagnostician
with data for visually and quantitatively determining the
perfusion.
[0016] In order to attain the aforementioned object, in the present
invention, the below-described medical facts are taken into
consideration.
[0017] 1) The cardiac chambers (the atriums and the ventricles) are
filled with blood.
[0018] 2) In general, blood perfusion disorder tends to occur on
the endocardial side of the myocardium, rather than on the
epicardial side thereof.
[0019] A contrast echo image obtained by use of ultrasound
diagnostic equipment involves the following problems.
[0020] a) In the case where a color image is obtained by means of a
power Doppler mode, the color of image regions corresponding to the
cardiac chambers may bleed toward an image region corresponding to
the myocardium.
[0021] b) The intensity of a contrast echo image varies in
accordance with a depthwise change in acoustic field intensity and
distance from the top.
[0022] c) The intensity varies from site to site due to noise.
[0023] The present invention solves the aforementioned problems by
dividing images of the myocardium and the cardiac chambers into
image regions which are considered to have the same acoustic field
intensity, and comparing and studying the image regions.
[0024] Specifically, the present invention provides a method of
analyzing a contrast echo image of the myocardium and a cardiac
chamber so as to determine the state of blood perfusion in the
myocardium, the contrast echo image being obtained by use of
ultrasound diagnostic equipment after injection of a microbubble
ultrasound contrast agent into blood vessels, and including a
myocardium image region corresponding to the myocardium and a
cardiac chamber image region corresponding to the cardiac chamber.
During the analysis, the contrast echo image is finely divided into
a plurality of calculation regions, each being considered to have a
uniform acoustic field intensity and covering a portion of the
myocardium image region and a corresponding portion of the cardiac
chamber image region. For each calculation region, a difference in
intensity between the myocardium image region and the cardiac
chamber image region is obtained, and the myocardium image region
is colored in accordance with the difference. This is the basic
procedure of a blood volume analyzing method of the present
invention which utilizes a myocardial blood volume map (MBVM). The
MBVM refers to a map showing distribution of blood volume within
the myocardium; i.e., an image of the myocardium which is colored
in accordance with the state of blood perfusion within the
myocardium.
[0025] In the present invention, the state of blood perfusion
within the myocardium is quantized by the following method. In the
blood volume analyzing method using an MBVM, a power P is first
obtained by the following equation:
10 log.sub.10P=I.sub.1-I.sub.2
[0026] where I.sub.1 represents the intensity of a myocardium image
region, I.sub.2 represents the intensity of a cardiac chamber image
region.
[0027] Subsequently, a current blood volume V (unit: ml/100 g:
blood volume) in the myocardium in a region of interest is
calculated by the following equation:
V=100.times.P (ml/100 g).
[0028] The present invention employs the following three methods in
order to obtain an MBVM. First, an equal-interval sliding window
method will be described. On an original contrast echo image, the
endocardial, which is a boundary between the myocardium and a
cardiac chamber, and the epicardial are traced to thereby form an
endocardial line and an epicardial line, respectively, and then an
inner line and an outer line are drawn on the inner and outer
sides, respectively, of the endocardial line in such a manner that
each of the inner line and the outer line extends along the
endocardial line while separated therefrom by a distance d2 (1 to
20 mm).
[0029] Subsequently, two boundary lines (acoustic field lines) are
drawn at an interval d1 (1 to 20 mm) in order to obtain a region
considered to have a uniform acoustic field intensity. A region
surrounded by the endocardial line, the inner line, and the
acoustic field lines is defined as a cardiac chamber calculation
region, and a region surrounded by the endocardial line, the outer
line, and the acoustic field lines is defined as a myocardium
calculation region. The average intensity of the cardiac chamber
calculation region is subtracted from the average intensity of the
myocardium calculation region to thereby obtain a luminous
difference. Subsequently, a color corresponding to the luminous
difference is selected from a color table, and the entirety of a
myocardium image region sandwiched between the acoustic field lines
is colored with the selected color.
[0030] The above-described difference calculation is repeated,
while the two acoustic field lines are shifted by the interval d1.
The reason why the interval of the acoustic field lines is set to
d1 and the average intensity of each calculation region is used is
to minimize influence of noise. Further, the color of the entire
myocardium image region is colored in accordance with the luminous
difference of the myocardium calculation region in consideration of
the facts that in some cases precise intensity data are not
obtained in the vicinity of the epicardial, and that a blood
perfusion disorder tends to occur from the vicinity of the
endocardial.
[0031] In an overlap sliding window method, a contrast echo image
is finely divided into calculation regions in a manner that is
basically the same as that used in the equal-interval sliding
window method. However, the overlap sliding window method differs
from the equal-interval sliding window method in the following
points. In order to further reduce the influence of variation in
acoustic field intensity, every time the intensity difference
between the myocardium calculation region and the cardiac chamber
calculation region is calculated for coloring the myocardium
calculation region, the acoustic field lines are shifted by a small
distance d3 (e.g., d3=0.5 mm) along a vertical axis perpendicular
to the acoustic field lines. In this method, since the shifting
distance is small, the calculation regions partially overlap. The
overlap portion of the preceding calculation region is colored by a
new color corresponding to the calculation value (intensity
difference) of the succeeding calculation region. The reason while
the vertical dimension of each calculation region is set to d1
despite the shifting distance being set to d3, which is smaller
than d1, is to eliminate influence of noise (the influence of noise
increases as the area of each calculation region decreases).
[0032] In a pixel-by-pixel method as well, a contrast echo image is
finely divided into calculation regions in a manner that is
basically the same as that used in the equal-interval sliding
window method. However, the pixel-by-pixel method differs from the
equal-interval sliding window method in the following points. That
is, the intensity is calculated for each of pixels within a region
surrounded by the endocardial line, the epicardial line, and two
acoustic field lines; the difference between the intensity of each
pixel and the average intensity of the cardiac chamber calculation
region is obtained; and each pixel is colored by a color
corresponding to the difference. This method enables evaluation of
an original contrast echo image as it is within a range from the
endocardial to the epicardial of the myocardium. However, a portion
of the myocardium image region containing noise or an unclear
portion in the vicinity of the epicardial are displayed as they
are.
[0033] In the case where a color image of the heart is obtained by
means of the power Doppler method, the color of the cardiac chamber
image region (blooming) often bleeds into a myocardium image
region, which hinders accurate measurement of the myocardium. In
order to eliminate the influence of such color bleeding, there is
provided means for drawing first and second buffer lines on the
inner side and outer side, respectively, of the endocardial line in
such a manner that each of the buffer lines extends along the
endocardial line while being separated from the endocardial line by
a predetermined distance d4, and defining, as calculation regions,
a region surrounded by two acoustic field lines, the first buffer
line, and the inner line, and a region surrounded by the two
acoustic field lines, the second buffer line, and the outer line.
The value of the distance d4 can be adjusted, or increased and
decreased in increments of 0.5 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Various other objects, features and many of the attendant
advantages of the present invention will be readily appreciated as
the same becomes better understood by reference to the following
detailed description of the preferred embodiment when considered in
connection with the accompanying drawings, in which:
[0035] FIG. 1 is a diagram of an ultrasound scanning beam showing a
method of obtaining contrast echo images by use of an ultrasound
probe;
[0036] FIG. 2 is a schema showing contrast echo images;
[0037] FIG. 3 is a cross sectional view of the heart (the
myocardium and a cardiac chamber) to be used for explaining an
equal-interval sliding window method of the present invention (an
example in which the heart is equally divided in the depth
direction by use of straight lines);
[0038] FIG. 4 is a cross sectional view of the heart (the
myocardium and the cardiac chamber) to be used for explaining a
coloring method used in the equal-interval sliding window method of
the present invention;
[0039] FIG. 5 is a cross sectional view of the heart (the
myocardium and the cardiac chamber) to be used for explaining an
equal-interval sliding window method of the present invention (an
example in which the heart is equally divided in the depth
direction by use of arcs);
[0040] FIG. 6 is a cross sectional view of the heart (the
myocardium and the cardiac chamber) to be used for explaining a
coloring method used in an overlap sliding window method of the
present invention;
[0041] FIG. 7 is a cross sectional view of the heart (the
myocardium and the cardiac chamber) to be used for explaining
another example of the sliding window method of the present
invention in which, for the sake of finer coloring, the myocardium
is finely divided into small calculation regions;
[0042] FIG. 8 is a cross sectional view of the heart (the
myocardium and the cardiac chamber) to be used for explaining a
pixel-by-pixel method of the present invention;
[0043] FIG. 9 is a cross sectional view of the heart (the
myocardium and the cardiac chamber) to be used for explaining a
method of eliminating color-bled portions from calculation regions,
which method is used in the coloring method of the present
invention; and
[0044] FIG. 10 is a diagram showing an image of the heart (the
myocardium and the cardiac chamber) colored by the coloring method
of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0045] An embodiment of the present invention will be described
with reference to the appended drawings. In the present invention,
the state of blood perfusion of the myocardium is analyzed on the
basis of contrast echo images obtained by use of ultrasound
diagnostic equipment. A computer unit is employed as hardware for
analysis. Contrast echo images obtained by use of ultrasound
diagnostic equipment are stored in a high-capacity external storage
medium such as a CD or MO, and image analysis is performed by use
of the thus-stored images.
[0046] FIG. 1 shows scanning of the body by use of an ultrasound
probe. Ultrasound beams b1, b2, . . . bn which are transmitted
(actually, continuously transmitted) from a tip portion 210 of an
ultrasound probe 2 applied to the body surface are reflected by an
object, and the thus-reflected waves are received by the probe 2.
The thus-received waves are converted to a digital contrast echo
image, which is then stored in a memory or a storage device. In
FIG. 1, an upward direction corresponds to a direction toward the
body surface, and a downward direction corresponds to a direction
toward the center of the body. The received reflection waves (echo)
are converted into intensity data, and the data are transformed
into color image data in the case of, for example, the power
Doppler method. The same acoustic field intensity (ultrasound
intensity) is attained at locations a distance (r) away from the
tip portion 210. In FIG. 1, reference numeral 10 denotes a line
connecting locations having the same acoustic field intensity
(hereinafter, the line will be referred to as an "acoustic field
line").
[0047] In an ultrasound echo image, intensity decreases depthwise
(toward the downward direction in FIG. 1). Meanwhile, the energy of
ultrasound waves becomes the highest in the vicinity of the focal
point. Therefore, when an echo image is displayed by use of the
absolute values of intensity data as in the case of a conventional
technique, the resultant image fails to be correctly evaluated. In
the present invention, every portion of an image region
corresponding to the myocardium (hereinafter referred to as a
"myocardium image region") is colored in accordance with the
difference in intensity between that portion and a corresponding
portion of an image region corresponding to the cardiac chamber
(hereinafter referred to as a "cardiac chamber image region"),
these portions corresponding to portions of the myocardium and the
cardiac chamber which are considered to have the same acoustic
field intensity. A specific procedure for coloring the myocardium
image region will be described below. The below-described procedure
can be applied to a monochromic image obtained by means of the
third harmonic method, the 1.5 harmonic method, or a similar
method, as well as to an image obtained by means of the power
Doppler method. This is because analysis is performed by use of
merely intensity data.
[0048] FIG. 2 shows an example of a contrast echo image of the
myocardium and a cardiac chamber. Such a contrast echo image is
obtained in the form of a color image (in the case of the power
Doppler method), or in the form of a monochromic image (in the case
of the third harmonic method or the 1.5 harmonic method). The
contrast echo image is captured at the below-described timing. A
microbubble ultrasound contrast agent is injected through the
veins. Although the contrast agent may be injected through the
arteries, in general, the contrast agent is injected through the
veins, since intravenous injection is less invasive. The contrast
echo image is captured at the timing when the contrast agent is
considered to have reached a region of interest and the region is
considered to have been sufficiently filled with the contrast
agent. In the case where a contrast echo image of the heart is
captured, the contrast echo image can be captured at a timing as
determined on the basis of the waveform of an
electrocardiogram.
[0049] Some ultrasound diagnostic equipment capture two images of
the region of interest at such a timing. The first image is
captured immediately after the region of interest has been filled
with the contrast agent, and the second image is captured when a
predetermined period of time (about 100 msec) has elapsed after the
capture of the first image. When ultrasound waves hit the
ultrasound contrast agent containing microbubbles, most of the
microbubbles vanish. Therefore, through comparison between the
first and second images, the state of noise of the first image can
be determined, along with whether the ultrasound contrast agent
fills the region of interest as expected. For example, when the
first and second images are identical with each other, the first
image may have been captured before the ultrasound contrast agent
had reached the region, which means that the captured images cannot
be used for analysis. The left-hand image in FIG. 2 is the
first-captured image, and the right-hand image in FIG. 2 is the
second-captured image.
[0050] FIG. 3 shows an image which corresponds to that of FIG. 2
and on which contours of the endocardial and the epicardial and
boundary lines (acoustic field lines) are drawn. The contours are
manually input by use of a suitable device such as a mouse. A
contour representing the endocardial will be referred to as an
"endocardial line," and a contour representing the epicardial will
be referred to as an "epicardial line." Coloring of a myocardium
image region in an equal-interval slide window method is performed
by the following steps:
[0051] 1) On an original contrast echo image, a region
corresponding to the endocardial 111 and a region corresponding to
the epicardial 112 are traced to thereby form an endocardial line
and an epicardial line, respectively. A region between the
endocardial line and the epicardial line corresponds to the
myocardium 110. This tracing step is performed manually; however,
the remaining steps are automatically performed by means of a
program.
[0052] 2) An inner line 11 and an outer line 12 are drawn on the
inner and outer sides, respectively, of the endocardial line 111 in
such a manner that each of the inner line 11 and the outer line 12
extends along the endocardial line 111 at a distance d2
therefrom.
[0053] 3) Vertical lines (horizontal lines in FIG. 3) 10.sub.k (k=0
to n, where n represents the number of divisions) are drawn at
intervals d1, starting from the cardiac apex (the tip portion 210
shown in FIG. 1), in the depth direction (the downward direction in
FIG. 3). Calculation is performed under the assumption that the
acoustic field intensity is constant within a region sandwiched
between two adjacent vertical lines drawn at the interval d1.
Therefore, the vertical lines 10 will be referred to as
"equi-sound-intensity contours" or simply referred to as "acoustic
field lines." Regions surrounded by the endocardial line 111, the
inner line 11, the outer line 12, and the acoustic field lines 10
are calculation regions. An average intensity A.sub.k of a region
(myocardium calculation region) surrounded by the endocardial line
111, the outer line 12, the acoustic field line 10.sub.k, and the
acoustic field line 10.sub.k+1 and an average intensity B.sub.k of
a region (cardiac chamber calculation region) surrounded by the
endocardial line 111, the inner line 11, the acoustic field line
10.sub.k, and the acoustic field line 10.sub.k+1 are obtained.
[0054] 4) The difference .DELTA..sub.k in intensity between the
myocardium calculation region and the cardiac chamber calculation
region is calculated as follows.
.DELTA..sub.k=A.sub.k-B.sub.k
[0055] where A.sub.k represents the average intensity of the
myocardium calculation region, and B.sub.k represents the average
intensity of the cardiac chamber calculation region. A color
corresponding to the difference .DELTA..sub.k is selected from a
color table c, and the entirety of a myocardium image region
sandwiched between the acoustic field line 10.sub.k and the
acoustic field line 10.sub.k+1 is colored with the selected color.
A hatched area in FIG. 4 represents the myocardium image region
colored with the selected color.
[0056] 5) The above steps are repeated, while the calculation
regions are shifted (in increments of one region in the vertical
direction in FIG. 3), so as to calculate the difference and to
color the corresponding myocardium image region, until the entire
myocardium image region is colored.
[0057] In the present embodiment, the interval d1 is set to 1 mm to
20 mm, and the distance d2 is set to 1 mm to 20 mm. The reason why
the interval d1 is set to 1 mm to 20 mm is that the acoustic field
intensity can be considered constant within each of regions formed
through division at the intervals d1. As shown in FIG. 1,
equi-sound-intensity contours are represented by arcuate lines 10
centered at the tip portion 210. However, in the example shown in
FIG. 3, the equi-sound-intensity contours are approximated by
straight lines. When the acoustic field intensity in each region is
required to have a higher level of uniformity, as shown in FIG. 5,
an echo image is divided depthwise by use of arcuate lines centered
at the tip portion 210. Since calculation regions (A.sub.k and
B.sub.k) to be compared with each other are narrow and adjacent to
each other, no practical problem arises even when each region
surrounded by straight lines is assumed to have a uniform acoustic
field intensity. However, since the region in the vicinity of the
cardiac apex (the tip portion 210) has a large area, as shown in
FIG. 3, the cardiac apex region is divided into n subregions by
radial lines extending outwardly from a point 0. FIG. 3 shows an
example in which the cardiac apex region is divided into 4 to 8
subregions. In this case, the difference A.sub.nj between the
intensity A.sub.nj of a subregion in the myocardium calculation
region and the intensity B.sub.nj of a corresponding subregion in
the cardiac chamber calculation region is calculated as
follows.
.DELTA..sub.nj=A.sub.nj-B.sub.nj (j=1 to 4)
[0058] Next, an overlap sliding window method will be described.
Although the overlap sliding window method is basically identical
with the equal-interval sliding window method, it differs from the
equal-interval sliding window method in the manner of sliding of
calculation regions. Specifically, as shown in FIG. 6, the
myocardium and cardiac chamber calculation regions are determined
in the same manner as in the equal-interval sliding window method;
however, the calculation regions are shifted along the endocardial
line 111 in increments of a small distance d3 (e.g. 1 mm). This
enables fine correction of a variation in acoustic field intensity
in the depth direction.
[0059] The reason why the depthwise dimension of each calculation
region is not set to the distance d3 is to eliminate influence of
noise as described in the summary section (when generation of noise
occurs locally, influence of such noise can be suppressed through
averaging of image data in a region of a large area (although the
area measures 20.times.20 mm at most)). In the illustrated example,
the calculation is started from the deepest calculation regions.
However, the calculation may be started from the shallowest
calculation regions. This is a matter of programming, and the
present invention encompasses both cases.
[0060] The reason why the color of a myocardium image region
between adjacent acoustic field lines is determined through
calculation performed for a corresponding myocardium calculation
region in the vicinity of the endocardial line 111 is as follows.
As described previously, in some cases the image is not clear in
the vicinity of the epicardial, and a blood perfusion disorder
tends to occur from the vicinity of the endocardial. Therefore, a
blood perfusion disorder can be determined even when the color of a
myocardium image region between adjacent acoustic field lines is
determined in the above-described manner.
[0061] However, as shown in FIG. 7, each myocardium image region
between adjacent acoustic field lines may be divided into two
regions; i.e., an endocardial subregion (A.sub.k) and an epicardial
subregion (A.sub.k'), and calculation for coloring may be
individually performed for each of the endocardial subregion
(A.sub.k) and the epicardial subregion (A.sub.k'). Moreover, each
myocardium image region between adjacent acoustic field lines may
be finely divided into subregions along the thickness direction of
the myocardium (the horizontal direction in FIG. 7). In either
case, the intensity of each subregion is compared with the
intensity B.sub.k of the cardiac chamber calculation region in
order to obtain an intensity difference.
[0062] Next, a pixel-by-pixel method will be described. As to a
method of determining calculation regions, the pixel-by-pixel
method is basically identical with the equal-interval sliding
window method; however it differs from the equal-interval sliding
window method in that each myocardium calculation region is
expanded to extend between the endocardial and the epicardial, and
that difference calculation is performed on the pixel-by-pixel
basis. FIG. 8 shows calculation regions defined in the
pixel-by-pixel method. Although each cardiac chamber calculation
region is the same as that defined in the equal-interval sliding
window method, a corresponding myocardium calculation region is a
region surrounded by corresponding acoustic field lines 10.sub.k
and 10.sub.k+1, the endocardial line 111, and the epicardial line
112. A calculation for coloring is individually performed for each
pixel within the expanded myocardium calculation region. The
difference .DELTA..sub.kj in intensity between each pixel in the
myocardium calculation region and the cardiac chamber calculation
region is calculated as follows.
.DELTA..sub.kj=A.sub.kj-B.sub.k
[0063] where A.sub.kj represents the intensity of the j-th pixel in
the myocardium calculation region, and B.sub.k represents the
average intensity of the cardiac chamber calculation region. A
color corresponding to the difference .DELTA..sub.kj is selected
from the color table, and the j-th pixel in the myocardium
calculation region is colored by the selected color. This enables a
diagnostician to accurately determine the state of the myocardium
on the basis of color. However, when the myocardium calculation
region contains noise, noise affects calculation for corresponding
pixels, and these pixels are colored accordingly.
[0064] Above, three coloring methods have been described.
Incidentally, in a color image obtained by, for example, the power
mode method, there occurs blooming, which is a phenomenon that
color of a cardiac chamber image region blooms into a myocardium
image region while passing through the endocardial line. Therefore,
influence of such color blooming must be eliminated. In the present
invention, as shown in FIG. 9, buffer lines b1 and b2 are drawn on
the inner side and outer side, respectively, of the endocardial
line 111 in such a manner that each of the buffer lines b1 and b2
extends along the endocardial line 111 while being separated from
the endocardial line 111 by a distance d4. A region surrounded by
corresponding acoustic field lines 10.sub.k and 10.sub.k+1, the
buffer line b1, and the inner line 11 is defined as a cardiac
chamber calculation region, and a region surrounded by
corresponding acoustic field lines 10.sub.k and 10.sub.k+1, the
buffer line b2, and the outer line 12 is defined as a myocardium
calculation region. Thus, influence of blooming can be eliminated.
Needless to say, in the case of the pixel-by-pixel method, the
myocardium calculation region extends between the buffer line b2
and the epicardial line. Notably, the value of the distance d4 can
be adjusted; specifically, increased and decreased in increments of
0.5 mm, and the buffer lines b1 and b2 are automatically depicted
on the basis of the adjusted value of the distance d4.
[0065] FIG. 10 shows an image of the heart colored by the
above-described method. Colors of subregions in the myocardium
image region represent the state of blood perfusion within the
myocardium. The darker the color, the worse the perfusion state.
Accordingly, a diagnostician can graph the state of perfusion by
viewing the colored image.
[0066] The above-described coloring methods enable visual
indication of the blood perfusion state. However, numerical
indication of the blood perfusion state is also possible. That is,
actual blood volume can be calculated as follows.
[0067] First, the power P of a region of interest of a contrast
echo image is determined. The power P represents a ratio of the
blood volume in the myocardium to the blood volume (100 ml/100 g)
in the cardiac chamber (during actual observation, intensity is
measured; however, intensity can be treated as blood volume,
because intensity is proportional to microbubble concentration,
which is proportional to blood volume). The power P can be obtained
from the following equation:
10 log.sub.10P=I.sub.1-I.sub.2 Eq. 1
[0068] where I.sub.1 represents the intensity of the myocardium
image region, I.sub.2 represents the intensity of the cardiac
chamber myocardium image region, which can be obtained from the
region of interest of the contrast echo image. I.sub.1 and I.sub.2
correspond to the above-described A.sub.k and B.sub.k,
respectively.
[0069] Accordingly, the blood volume V (unit: ml/100 g) in the
myocardium in the region of interest in an ordinary state can be
calculated as follows:
V=100.times.P (ml/100 g)
[0070] For example, in the case where the intensity difference
.DELTA.=-10 dB (.DELTA.=I.sub.1-I.sub.2), from Equation 1, the
power P can be obtained as follows.
P={fraction (1/10)}
[0071] Therefore, the blood volume V in the region of interest can
be obtained as follows.
V=100.multidot.P=10 ml/100 g
[0072] Since the blood volume in the myocardium is less than that
in the cardiac chamber, the thus-obtained blood value V is
normal.
[0073] Diagnosing blood perfusion within the myocardium from a
contrast echo image requires a high level of skill and experience,
and is difficult even for a medical specialist. In addition, such a
contrast echo image involves various problems such as inclusion of
noise, depthwise attenuation of acoustic field intensity (intensity
of ultrasound waves), and increased intensity in the vicinity of a
focal point. However, since conventional analysis methods have not
taken these problems into consideration, blood perfusion diagnosis
has depended heavily on experience.
[0074] The present invention has solved the above-described
problems by obtaining the difference between the intensity of a
cardiac chamber image region and the intensity of a myocardium
image region, which regions have the same acoustic field intensity.
Specifically, even when the acoustic field intensity attenuates
depthwise or locally increases in the vicinity of the focal point,
problems attributable to variations in intensity among locations
caused by the attenuation and local increase are solved by
calculating the difference in intensity between a cardiac chamber
image region and a myocardium image region, which regions have the
same acoustic field intensity and serve as calculation regions.
Moreover, the problem of local noise generation is solved by
obtaining the average intensity of each calculation region (narrow
region which is considered to have generally a uniform acoustic
field intensity). Furthermore, the problem of color bleeding which
would otherwise occur in a color image produced by the power mode
method is solved through provision of buffer lines. For these
reasons, a colored image of the myocardium obtained in accordance
with the analysis method of the present invention enables a
diagnostician to easily grasp the state of blood perfusion in the
myocardium by observing the colored image of the myocardium. This
is because colors in the color table represent different degrees of
blood perfusion.
[0075] When the above-described analysis is to be performed, a
diagnostician (usually, a doctor) is required to draw an
endocardial line and an epicardial line of the myocardium, which,
however, is a simple step. The remaining steps are performed by
means of a program. Therefore, any person can easily use the
analyzing method and can determine the state of blood perfusion in
the myocardium. Use of the blood volume calculation equation of the
present invention enables quantitative determination of the state
of blood perfusion in the myocardium, because the blood volume at
the region of interest can be obtained in the form of a numerical
value.
[0076] The present invention provides three coloring methods; i.e.,
the equal-interval sliding window method, the overlap sliding
window method, and the pixel-by-pixel method. In the sliding window
methods, the state of blood perfusion in the vicinity of the
endocardial is represented by use of colors. Use of the
equal-interval sliding window method realizes high calculation
speed. Use of the overlap sliding window method realizes continuous
representation of the state of blood perfusion in the myocardium.
In the sliding window methods, each myocardium image region between
acoustic field lines is colored in accordance with the state of
blood perfusion in the vicinity of the endocardial. However, as
described in relation to the above embodiment, when each
calculation region is finely divided along the thickness direction
of the myocardium (the direction from the endocardial toward the
epicardial) for calculation of intensity differences, the state of
blood perfusion, which may vary in the direction from the
endocardial toward the epicardial, can be accurately displayed.
[0077] However, as described previously, in some cases a clear
image is not obtained in the vicinity of the epicardial. Therefore,
at the present time, the sliding window methods, which color each
myocardium image region in accordance with the intensity difference
of a corresponding calculation region in the vicinity of the
endocardial, raise no practical problems. Specifically, a blood
perfusion disorder tends to occur from the vicinity of the
endocardial, and therefore, when the vicinity of the endocardial is
analyzed accurately, no significant medical problem arises.
[0078] The pixel-by-pixel method does not involve the
above-described problems, because the entirety of a myocardium
image region is evaluated on the pixel-by-pixel basis. That is, an
original contrast echo image can be evaluated accurately from the
endocardial to the epicardial. However, since a portion of the
myocardium image region containing noise or an unclear portion in
the vicinity of the epicardial is evaluated as it is for coloring,
the pixel-by-pixel method cannot be said to be superior to the
sliding window methods at the present. Therefore, the
pixel-by-pixel method can be effectively used as a method for fine
evaluation when a contrast echo image which does not contain noise
and which is clear to the vicinity of the epicardial is
obtained.
[0079] Once a contrast echo image is available (is obtained), the
analysis-display method of the present invention can be practiced
by use of a commonly used personal computer. Therefore, any person
can introduce the analysis-display method in the form of low-cost
image analysis software. Further, since operation is very simple,
the analysis-display method is an effective tool for doctors who
are unaccustomed to using personal computers.
[0080] Obviously, numerous modifications and variations of the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the present invention may be practiced otherwise than as
specifically described herein.
* * * * *